172 



ZOOLOGY 



best known from their "bones" or rudimentary shells 



\j 



embedded in the mantle, are inhabitants of deeper 



waters. They have shorter bodies 

 than the squids, and their arms are 

 eight in number instead of ten 

 as in the case of the squids. In both 

 groups locomotion is effected by the 

 reaction to a stream of water which 

 is taken into the mantle chamber 

 at the edges of the mantle and 

 is forced out through the " funnel," 

 which lies between the eyes in Fig. 

 1(33. 



The shell is rudimentary in both 

 the squids and the cuttlefishes, and 

 like that of the slug is embedded in 

 the mantle. Even in Spirilla the 

 shell, though coiled and containing a 

 siphon, is completely covered by the 

 skin of the animal. There are two liv- 

 ing genera of Cephalopoda which have 

 an external shell. One is an ally of 



FIG. iitt. L<jiit/<j Peaia, the cuttlefishes the paper nautilus 



eastern squid. One-third t ^ii'i--C i n 



nat. size. From Rathbim. O1> Argonauta? which IS found in all 



tropical seas. The other is the sole 



survivor of a once abundant group. This is the Pearly 

 Nautilus of the Indian and Pacific oceans. Its shell is di- 

 vided into water-tight compartments, in the last formed 

 of which the animal lies. It keeps its attachment to 

 the shell by means of a central strand of tissue the 

 siphon (Fig. 164, s). Allied to the Nautilus is the huge 



a sailor in the Argo. 



